Boeing’s 737 in another pickle

By Bjorn Fehrm

October 1, 2019, ©. Leeham News: The FAA has issued an Air Worthiness Directive (AD) for high time Boeing 737 NGs, requiring immediate inspections for cracks in their wing attachments called pickle forks.

The cracks were discovered on high time aircraft which were torn down for conversion to freighters. The affected 737 types are NG only; the MAX and Classic have a different wing attachment design. The P-8 Poseidon, a derivative of the NG, also is unaffected.

Cracks develop at one third expected lifetime.

The issue was first reported by Charlie Harger of  KOMO radio station in Seattle. He reported cracks were found in the reinforced fuselage frames, which are the main attachments of the wing to the 737 NG fuselage. The frames are placed at the forward and rear wing spars where their lower parts are used to transfer most of the load from the wing into the aircraft’s fuselage.

These frames have reinforcements called “pickle forks,” as the reinforcements attached to the frames look like a cocktail pickle fork, Figure 1. In total the aircraft has four pickle forks, one on each side of the forward attachment frame and one on each side of the rear attachment frame.

Figure 1. 737 NG pickle forks reinforce the frames attaching to the forward and rear wing spars. The influence of a flexing wing can be seen in the picture. Source: Leeham Co based on a patent drawing.

The wing is also attached with the frames meeting the wings top surface between these attachment frames. These fuselage frames are not designed to carry much load, however, as the wing’s top surface is not a good attachment point for the fuselage.

The attachment frames with their pickle fork reinforcements are a so-called safe life design (see below for more on this) which means they shall last well above the life of the aircraft, in this case, 90,000 flights. The cracks were found on ex-Jet Airways of India passenger planes which were converted to freighters for Amazon with more than 33,500 flights behind them. When doing these conversions the airframes are stripped bare and the cracks were found.

Affected aircraft

The issued AD affects Boeing 737 NG aircraft with over 22,600 flight cycles (flights). These shall be inspected within one year. For aircraft with more than 30,000 flight cycles, the inspection shall be completed within one week from the effective date of the AD.

A typical 737 NG flies 2,000 flights per year so the AD affects aircraft aged 11 years or more. From 737 NGs of 15 years or more, the AD is critical, the inspections which are done with a boroscope and takes one hour shall be done within a week or the aircraft is no longer allowed to fly.

How dangerous are cracks in our airliners?

Cracks due to age are found all the time in our airliners. As they conduct the takeoff, flight and landing (a flight or Flight Cycle), their structures are subject to stresses which fatigue the metal over time. The airframes are designed to handle this metal fatigue either with redundant load paths, a so-called fail safe design, or a design with very low stress levels giving very long life, a so-called safe life design.

The prescribed maintenance of the aircraft has inspections of the structure at regular intervals to find any issues like cracks which develop in the airframe before these become dangerous. The major tear down for the 737 NG, the so-called heavy checks, are at six and 12 years after entry into service. These are specially designed to search for cracks and other issues affecting the aircraft’s structure.

The 737 NG pickle forks are a safe life design, according to our information. This is what makes the cracks worrying. They shouldn’t be there on an airframe with only 37% of its design life behind it.

What can be the cause of the cracks? Here some first thoughts before we have more information from Boeing and the FAA:

  • It could be an aluminum alloy quality problem, restricted to a small population of 737 NGs. The alloy, 7075T3, is a well know aircraft alloy and Boeing has material tracking data to single out the production aircraft which would be affected by such a problem.
  • It could be a production problem where the attachment of the frames to the wing (more specifically to its center wingbox) causes permanent stress levels which should not be there. This shortens the fatigue life of this particular part. This can be a problem for certain production numbers or a more widespread 737 NG issue.
  • Finally, it can be a design problem. The local stresses for the 737 NG are higher in this part than envisaged. This is perhaps the least likely cause, as the design has been tested by several full-scale tests for the stress levels and the fatigue ground specimen has cycled the design for well over 90,000 cycles without this kind of issues.

These are the kind of questions Boeing and the FAA are seeking the answer to. Important information in this search for the root cause is the results of the ordered inspections. So far three aircraft have had cracks. Fifteen aircraft had been inspected through yesterday.

The flight spectrum and production numbers of the aircraft with cracks will be an important lead to the root cause of the issue and what further actions shall be taken.

151 Comments on “Boeing’s 737 in another pickle

  1. There are so many NGs with way more cycles that went through at least two D-Checks. So the problem – if it was common to all B737NG – should have emerged before.

    • Whats really needed is photos and drawings marked up with location etc.
      The press says ‘ cracks clear thru ‘- which is NOT definitive

      Could be cracks around holes, or cracks thru the ‘crotch’ of the pitchfork or in the web portion of the pickle fork.
      I suggest that before all the pundits decide to redesign or explain what really happened- WAIT for more specific data

      • Just curious, are there any type of cracks where you believe there is no cause for concern?

        Personally, I cannot think of any. Judging by what has been reported on the AD, Boeing seems to be of the same opinion.

        • it really depemds on where cracks are- between two holes on a thin web for attaching a non critical item, orthru a major part such as the ‘crotch’ of the pickle fork.

          Not all cracks go ‘ thru’ a part – and depending on a bunch of facts as to loacation, progression, they can be deferred to the next major check.

          • The drawing conveys the loading pattern of the fork. I guess it transfers the landing load from landing gear to wing spar and from there through the respective pickle forks to the fuselage bulkhead..So how many landings has the stress engineer allowed this fork to endure before it starts cracking? What is the failure tolerance zone? Is it a secret?, and why FAA is not talking about redesigning this wing to fuselage joint. As a passenger, I need to know.

    • The issue was found not doing a D-Check but during a freighter conversion. It is is likely that more the structure gets exposed in a conversion than in a D-Check.

  2. If, on inspection, this is determined to be common, it has the potential to be quite an issue for the large 737 LCCs, who are being affected by the max grounding also. Further route cuts etc. Also looks like a difficult fix? The pickle dork looks to be quite embedded and large.

    Anywho… Interesting to see how this turns out.

  3. Even if it was only 1% with this issue, that’s too many. Remember it wasn’t found at D check but a more extensive internal tear out for cargo conversion.

    • d-check.
      isn’t that were you time a conversion for?

      ( 20 years, 7300 days, 5 cycles per day -> 35,000 cycles
      high cycles should translate to less flight hours.)

    • As the D checks look for just this type of issue the fleet looks safe.

      Not a given and the guidance is going to tell us a lot (checks at the higher cycles)

      As long as the pickles ahve been looked at in the heavy checks, then same assumption on the 20k.

      If more data surfaces (pun) that there are more in the system and what hours, then that guidance will shift immediately.

      This could also be a one airline issue, either the maint practices or the use and possible abuse of the aircraft.

  4. Presumably any aircraft with cracks will be grounded?If so, we’ll know just how bad this is within a week.

  5. “The affected 737 types are NG only, the MAX and Classic have a different wing attachment design.”

    A deviation from the title topic. But.. :
    Any news byte about 737 in general seems to increase the number of distinct changes in design under the hood found on the MAX.

    How far reaching are these changes really? For up front a simple reengine that is surprising imho.

  6. Uwe,
    The Leap 1B engines are around 400 Kg heavier than the CFM-56, and of course are larger, and create more drag, and more thrust, so perhaps not surprising that there are structural changes.

    Anyone here that knows how the MAX wings are attached ? I’d be interested in specifics.

    Like you Uwe, I am wondering just how many more changes there have been from NG to MAX, and I also wonder if the FAA were made aware of these changes or did BA just self certify ?

    I do think that grandfathering needs to be looked at. The MAX is starting to look like quite a different aircraft to the NG.

  7. Bjorn,

    Do you know for certain if the really have/had a separate fatigue test rig for the NG? I thought that was one of the benefits of keeping the 737 name, the grandfathering of old certification activities to minimize new tests and analysis.

    On the other hand, when one redesigns the concept of the wing to fuse attachment, I would expect a full set of static and fatigue tests for that.

    But it does beg the question as to why they felt the need to change the design from classic to NG and then from NG to Max?

    One other possible cause could be parts that have not been manufactured to tolerance. I imagine that Boeing is inspecting the relevant parts to see if they are manufactured to tolerance. Of course, if this is the case, it does not look good for QC.

    Clever title to your article there.

    • Hi Aero Ninja,

      the NG came with a 100% new wing. The wing is larger, has a different load and wing moment which must be transferred into the fuselage, compared to the Classic. So I would say a new fatigue test specimen, at least for the wing including fuselage interface.

      • Looked around and could not find any hint. neither fatique nor ultimate load testing. ( lack of google foo? search results are obscured by this recent issue. flight global archive comes up empty too)
        You find imprints for 777 test specimen though.

        IMU care was taken to keep enough of the wing as is ( and the wingbox too ) to allow grandfathering and avoid retesting.
        Better idea where to look?

      • So we presume that as the MAX wing is different again, and the method of attachment to the fuselage is different to the NG, that BA would have performed an ultimate wing load test on a test fuselage, or is this not necessary due to grandfathering ?

        I can’t remember having seen any MAX fuselage created for structural testing, I thought that the first MAX built was intended for delivery to Southwest after testing ?

        • We can only assume its attached to the fuselage.

          As its the same wing likely the same attachment system.

          We won’t know if its been beefed up or not until the in depth comes out.

          Taking one from Bjrons Book of puns, Stay Tuned! (tuning fork, ala pickle fork from US reference)

      • I am late (on hols in wilds of Italy) but would add one item to your comprehensive list … others have raised manufacture. If a batch were machined by a different supplier from, assuming bare forging from the shape, then that would reduce the scale of things. Boeing must have all such details by serial number of the individual cracke d part. The fact that the AD is segregated by flight cycles makes this unlikely. Ditto design error … it would most likely have appeared as a problem a long time back.
        Great piece by the way. Thanks!

        • Mentioned was “multiple frames”. That is 3 or more.
          How many frames are in freighter conversion currently ( at that specific MRO company)?

          I’d be surprise if they even work on more than two at a time.
          That would hint that _every_ frame touched shows the same kind of damage!?

          • Looks like it is the Boeing freighter conversion divison ( for 737-800BCF) that found the defects !?

  8. In general it doesn’t worry me too much – not because it would be insignificant, au countraire, but because it has been found and addressed properly by AD and mandatory inspections.

    It is a problem because someone in someplace underestimated something – so check & replace & redesign if needed, but without panic.

    I would also would like to know if proper stress test & certification was done or it was written off on grandfathering-rights basis. I would be happy if @Bjorn would dig much deeper about it.

    If it is a grandfathering-rights issue it maybe would be a time to worry about 777x certification which have reengineerd fuselage construction, thinner ribs etc.

    • Keep in mind you can fail a test and still get certification’s with an approved correction.

      A380 broke the wing at 147%, but it broke where they predicted it would (just sooner) and they simply beefed it up and did not have to re-test.

      I am curious why this was not found in test as the tests go well past the life estimate of the air frame.

      The reality is you use your design tools and you have to believe the results but then you test to confirm how good the software is.

      Boeing has tended to be over design (with some issues like the fuselage skins on the 737 and 757 not so)

      Hawaiian blowout was a cycle type not envisioned though Boeing knew how they were using it and should have tested to that (corrosion is an imposible one to model in though)

      • Agree that could happen that some part would pass a test but than fail in live. But I’m interested if @Bjorn would deliver an information if Boeing even tested these pickle forks of changed design.

    • Whether or not we should worry or not depends on the result of this survey, and whether or not a convincing explanation can be found. If the cause remains a mystery, that’s a very big problem, especially if this trawl through the fleet reveals more examples. Still, by doing a bunch of inspections they can start to get a statistical picture of the extent and possible origin of the issue (manufacturing batch, design flaw, or whatever).

      Though it’s a good thing that it has been found, it has to be noted that this was luck, not the consequence of a designed inspection process. Sharp eyes by the crew doing that conversion work, and well done for reporting it.

      ***What They’ll Already Have Done***
      I’m presuming that by now the histories of the fault aircraft have been picked over meticulously. I wonder if unrecorded heavy landings are a possible cause? Though I suppose there’d be other signs of these elsewhere in the airframes.

      I’m also presuming that a materials quality analysis on the fault aircraft has already been done. If the material was found to be below standard they’d at least know by now which batches of material to be worried about and thus exactly which aircraft to look at.

      The fact that they’re doing a sweep of the whole fleet >20,000 cycles suggests to me that the material is up to scratch, and they’re looking for other causes.

      A more optimistic alternative is that they think they’ve identified a cause, and this sweep of the fleet is a way of confirming that it is indeed a problem limited to a batch.

      The cracks could be due to corrosion. However, I think that this would mean that the corrosion protection coatings were not up to scratch. This could be a fleet wide problem, possibly easily resolved with a brush and a tin of chromate?

      Failing all that, that means it’s a manufacturing fault, or a design fault.

      If it’s a manufacturing fault, then either the prescribed manufacturing process was inappropriate, or the manufacturing process wasn’t followed.

      Either way they’re screwed. A bad process means they’re all wrong. A process not followed correctly means they all have to be considered to be wrong until proven otherwise. I don’t know how you’d take an in-service aircraft and prove that its pickle forks were manufactured properly. It sounds difficult.

      ***Design Fault***
      If it’s none of the above, then the design is wrong and that’d be that for the NG, probably.

      737 operators must be thinking, “Whatever next?”.

      • Yes it is to worry that it hasn’t been discovered during planned inspection but by chance.

        I hope that data will arrive and will tell us what went wrong.

    • Is this why the cargo door failed under test or was a short circuit to the S2 switch that cargo door on United flight 811 in 1989 on it way to New Zealand with a loss of 9 lives.

  9. What will be the fate for those aircraft which are forund to have cracked pickle forks?

    Too complicated and/or too expensive to repair?

    • If (i) the MAX debacle had not happened and (ii) this only affected a small proportion of the NG fleet, I’d say probably, too expensive to justify repair – there almost certainly aren’t even jigs or methods existing for repair of these. So you’d be looking at maybe 7 figure sums each if they were limited to relatively few frames [considering then amortisation of engineering effort and tool build cost].

      But, given the shortage of lift due to (i) and the unknown constraint this would put on supply as a result of (ii) – at the moment I’d cover my bases and say, “not sure”.

      • It all depends on the value of the aircraft.

        A very old NG that is being converted to F is likely to be dumped as the economics of an F conversions assume standards of work based on previous.

        If some show up in active pax fleet then its a coin flip on the current MAX issue weighing in as to how value an NG Pax is at the moment.

        Bird in the hand sort of thing.

  10. Aero Ninja, I think your concern that parts have not been manufactured to tolerance, is covered in the original post, by the statement:- “it could be a production problem where the attachment of the frames to the wing (more specifically to its center wingbox) causes permanent stress levels which should not be there.” Components and assemblies not manufactured to tolerance are subject MRB repairs, which should result in a safe repair. If parts and assemblies were not to tolerance then as the original poster says, that could causes permanent stress levels (if not repaired correctly), which should not be there, thereby shortening the life of the component and assembly.

    • You assume that there was a repair and that the out of tolerance part was simply not bought off as acceptable by some liaison engineer.

      That happened to me once. I got called down to the shop floor and was told that the assembly couldn’t be put together. After about 15 minutes of looking at the paperwork, I found a concession for one of the fittings that was out of tolerance bought had been bought off with no rework.

      No wonder it didn’t fit!

  11. Was actually found during voluntary conversion from passenger to freight!This is alarming, passengers have almost certainly be flying around in unsafe aircraft. Information on Pprune suggests that the pickle forks were cracked all the way through. In my mind QC is the likely culprit. Hopefully this is a limited problem, as I would have expected to hear a lot more horror stories by now if it affected a large proportion of the fleet. It’s not a good idea for the entire world to rely so much on 2 types of aircraft. There are many reasons why the design might have been changed for the MAX ,but one of them might be inadequacy of the NG design.

    • Alaska 8.9 years average age,Ryanair 6.5,Southwest 11.8,United 10.4.I can’t see how Southwest can avoid a major problem.

    • Grubbie:

      Has there ever been a non voluntary conversion from Pax to Freight?

      Ok Airplane, if you don’t convert I am going to cut your tail off!

      • Someone said it was a mandatory inspection,it wasn’t and MRO network quotes Boeing as saying that this part isn’t ordinarily inspected at all.I find that hard to believe.

        • I can believe that it’s not routinely expected. With a bunch of design analysis and testing, one could get to the point where one is confident enough in the fatigue and load margins to trust it for the lifetime of the aircraft.

          Though that does place a heavy emphasis on getting the design spec right, on testing examples properly, and on manufacturing and assembly being correct. Any one small thing in all of that being wrong and, well, that’s a hidden problem.

  12. Are the MAX and P-8 not involved because they have a different structure or because they just have not hit those hours?

    • by far not old enough ( or don’t see the hours/cycles for the P8 )
      found in scope of NG freighter conversion.
      37k cycles is 20 years @ 5 cycles per day.

  13. The design of pickle forks mainly is to cover the attachment in the wing fwd & starboard areas to the fuselage. as such it can always be replaced periodically with new parts. They are not load carrying assembly. And by ultrasonic or visual inspection we can notice cracks. Already 737Max is burdened with software updations,hope Boeing will clear this also with non recurrence snags

    • The story does say they are load carrying:
      ‘The frames are placed at the forward and rear wing spars where their lower parts are used to transfer most of the load from the wing into the aircraft’s fuselage”

      • Yes, you need a wing torque reaction supports at the fwd an aft end of the wings to make sure the wing beams attachment bolts mainly take vertical shear load as the center of lift moves around.

    • Ah contraire, Mohan BJ, these pickle forks are Major load paths. They are the main load carrying members that carry vertcal wing loads into the fuselage.

  14. I’m sure the pickle forks cracked because they were flown by inexperienced non USAF alumni crew in India 🙂

    But seriously though the pickle fork issue was known for a while if you read the comments on pprune. If the max fiasco wouldn’t have happened nobody would be talking about this AD.

  15. Anyone know the AD number? I have looked on the FAA website and the last AD against Boeing was 2019-18-03 which is for a thrust reverser issue. No emergency ADs listed which is where I would expect to find an AD requiring compliance within a week.

  16. The P-8 Poseidon is not a derivative of the MAX but of the 737-800.

    • Correct, an NG derivative not MAX.

      It does not have winglets and it flie a totally different profile from commercial aircraft for its missions.

      We have to see what the failure is and then if there is any affect on the P-8 which does some low and slow but the majority of its hours are high cruise unlike a 737 PAx that is up and down a lot.

      The low and slow may be nasty enough to be an accelerate or a non issue.

      Until we know the cause we simply cna’t know

      • We will see how it works to hunt submarines from high altitudes. The P-8 did not get MAD-sensors which are far more effective in case the aircraft is closer to the target like the low flying P-3. Submarine hunting without MAD? We will see how effective this will be. Maybe some day in the future P-8 will have to fly far lower.

  17. I thought the heavy airframe checks for NGs were at 8, 10 and 12 years and then repeat. 6Y and 12Y are Airbus narrowbodies, I thought

  18. @ Scott:

    “…the MAX and Classic have a different wing attachment design… ”

    MAX has an almost identical pickle forks to the NG, only some of the picklefork thicknesses are different to accommodate the increased loads since the MAX is heavier than the NG. Wing to body joint is essentially the same, with the exception of minor thickness or fastener count increases. This does not mean the fatigue life for the MAX pickleforks are any better than the NG. It is highly likely that the issue is common between the MAX and the NG and maybe its not sensible to issue an AD for an already grounded model (i.e. the MAX).

    • If the Pickle Fork has been changed then the AD is not relevant and we are talking about 35k cycles.

      You simply put it in the test Q to see if the upgrade also corrected the issue.

      As its going to take 20 years to get there……………………

      The A380 flew with known wing issues that were deemed not an issue in the short term and those manifested fairly .

    • How can it be highly likely when we don’t even know the underlying cause?

      What is highly likely is that its 15 years ago that the 737NG was built.

      As its been tested well beyond the cycles we have seen, that says its highly like a non design issue.

      Data will come in and tell us.

      • Yeap, data will tell us what’s wrong with NG and then if MAX is in danger.

        There is no sense @AJones to address AD wider because all MAXs are new aircrafts, AD covers NGs with at least 20.000 cycles witch equals ~11 years of service.

        • @ Transworld
          I think you are missing the main point of my comment so let me clarify: the structural wing to body joint is essentially identical between NG & MAX. Different thickness is due to the MAX loads being higher than the NG, not because MAX has been designed to a higher fatigue life to cover this issue; MAX was designed to the exact same fatigue life requirements as the NG, and all else being equal (metallurgy, assembly), a MAX picklefork will be expected to have identical fatigue failure modes to that of an NG after the same number of cycles.
          Static testing is done on properly manufactured and within spec airframes. Tests don’t capture unusual deviations from the standard build process, which I think this may be.
          Worth noting that the 737 MAX production process is basically identical to that of the NG (common cert), and the 737 line in Renton has a notorious history of force-fitting out-of-tolerance structural parts when the need arises (i.e. using a hammer or whatever it takes, this has been covered by the press previously). This is done to reduce Takt time, scrap and machine shop reworks.
          I repeat: this is very likely a production issue that effects both the MAX and NG airframes intermittently. I hope they figure it out.

          FAA doesn’t care if the fleet is old or young. They file an AD for the types effected if/when the root cause analysis shows there is a common cause for concern, then the AD would be set to go into effect at a future date i.e. after X cycles or Y flight hours. That root cause analysis might take a bit longer, and in the meantime, they know the MAX fleet isn’t going anywhere.

          • @A Jones: “The 737 line in Renton has a notorious history of force-fitting out-of-tolerance structural parts when the need arises (i.e. using a hammer or whatever it takes, this has been covered by the press previously).”

            It has indeed been covered by the press. Here is the link for the video that explains what the situation was in 2010 and which may have some bearing on the latest NG problem:


          • Yes @AJones, but it is an AD for immediate inspection to find cracks that already exists or soon be. After investigation I think it will be a second one – more complete and more explanatory.

            You made good points anyway, but sometimes a small change results in different outcome, and sometimes not. At this stage I will wait for data that will start to arrive in a week.

          • hey norm hamel ?
            Normand Hamel
            October 1, 2019
            @A Jones: “The 737 line in Renton has a notorious history of force-fitting out-of-tolerance structural parts when the need arises (i.e. using a hammer or whatever it takes, this has been covered by the press previously).”

            Actually the video related to parts used and assembled in Wichita- not renton.

            Not to say Renton was perfect-

  19. Many people here are asking if a proper static test was carried for the fuselage. As others I assume they did.

    On the other hand I also remember that when we first heard of the grand-father clause it was mainly about the fuselage, which in case of a crash is not as robust as the A320’s. That is why the latter is heavier than the former and this gives a clear advantage to the 737.

    In fact we can see how weak the 737 fuselage is when it crashes. It clearly has a tendency to break into several parts, and depending where one sits the possibility of injury or death is obviously higher. That is the main reason why the regulations had been updated by the time the A320 was certified.

    That being said, the following question remains on my mind: Was a proper static test carried out? Because if indeed it was, would not the necessity of doing so void the grand-father clause for the fuselage?

  20. May I start by doing a comparison before addressing the article. I will use Airbus and Rolls-Royce and their newest products, the A350 and the Trent XWB.

    Airbus discovered that the rear fuselage frames were not strong enough to meet their safe life. There safe life design was to be greater than the safe life of the airplane. The discovery was made after certification.

    Rolls-Royce discovered that the front engune mounting was not strong enough to the safe life. The safe life design was to be greater thwn the safe life of the airplane. The discovery was made after certification.

    Airbus and Rolls-Royce informed the regulator, EASA, and ADs were issued. Standard procedure.

    But we come to the 737 NG. The customer finds the problem, not Boeing. This suggests Boeing stops doing tests and analysis once an airplane is certified. Other OEMs continue tests and analysis even though their products are certified.

    The pitch stability issue with regard to the A320 is another example. Airbus found it and reported it to EASA, who issued an AD.

    The point I’m making is that issues with Boeing airplanes are not found by Boeing. Other must find the issues. Then Boeing start arguing responsibility. The chief argument is pilot error.

    Something the new committee must think about for rumour on the street is Boeing want to do even less testing and analysis than they do now when they should be doing more.

    Expecting customers to find issues and then arguing the issue is never right. Boeing must find the issue before the customer does and fix it before the customer has problems.

    I will end with saying LNA have not got their definition of safe-life and fail-safe right. I’ll correct in another post.

  21. May I address safe-life and fail-safe.

    Safe-life is life of a part before it fails. All parts of an airplane have a safe-life. Most parts have a safe-life that exceeds the safe-life of the airplane. Many don’t. They are referred to as life limited parts, LLPs. LLPs are typical in engines, specfically the hot section. LLPs must be replaced before their safe-life expires.

    Fail-safe is a process of redundancy that protects againt a part failing before it’s safe-life expires. What does fail-safe do. It says if a part fails it’s responsibilty transfers to another part. That is called double or duplex redundancy. If the second part fails it’s responsibility transfers to a third part. That is called triple or triplex redandancy. And so on.

    In practice the parts involved in a fail-safe share responsibility, but each part has the ability to accept total responsibility.

    Fail-safe exists because engineers get it wrong. In my prior post, Airbus got it wrong with regard to rear fuselage frames. Rolls-Royce got it wrong with regard to the front engine mount.

    Boeing have got it wrong with regard to pickle forks. But be clear, the wing/fuselage join is fail-safe otherwise many, many people would be dead by now. The failure has been caught. But Boeing should have done the catching not the customer.

    I will end my addressing Boeing’s deceit when they say that when a sensor mis-match occurs MCAS turns itself off. We are being told that is duplex. No it is not. The pilots don’t know which of the sensors have failed. So they have a 50/50 chance. If they choose right they live, if wrong they die.

    EASA have picked up on this. In their PDF they made clear they have received no response to alpha vane integrity. In other words, it not good enough to have a 50/50 chance of picking which is right and which is wrong.

    As I have said before, error logic is necessary to isolate sensor failure and instruct accordingly.

    • Pablo:

      Generally that is correct but keep in mind its not just hot section engine parts that wear out. Engine oil has a life limit, filters do, bearings do hot or cold. hydraulic pumps wear out, cables wear out, jack screws wear out.

      I fully disagree on Boeing should have found this. All evidence says it was designed right, the materials were right and the testing was done right (and well past the point it should show up)

      You should bear in mind that Airbus did not know about the Wing Foot cracks on the A380 until the engine blew up in Singapore and it was revealed not as part of an inspection but the investigation and what they saw.

      Unfortunate the test articles are hand made and aircraft morph into production.

      GE got bit by that with the coating on the GENx shaft, there should have been no issue and no one involved saw it and ignored it but it was there.

      Arguably a second fuselage that is X % into the production stream should be tested. That is a regulatory role not a Boeing error.

      Arguably the standard of testing has worked in the past though that does not mean it should not be changed or amended. Its a costly process and should be a regulation though. Level playing field and all that.

      Twisting this into the current MAX issue does a real disservice. Any beating up on Boeing on the MAX is justified, it was beyond a gross failure.

      That does not mean all failures should have them strung up and whipped in the Sun. By twisting these together you do a disservice to a good post.

      • @TransWorld

        I believe you were responding to @Philip, and actually to his previous post.

      • TransWorld

        There are no absolutes. So sometimes things get past other OEMs. But the record now clearly shows Boeing is in a place of their own.

        With regard to twisting things into a 737 MAX issue. The proposals for MCAS do not represent duplex redundancy. To suggest it does is laughable.

        But then we come to grandfathering. If Boeing have used grandfathering to avoid a full structural test on the 737 MAX then Boeing now have additional problems to address. This is especially true as the wing/fuselage join is not the same, as other commentators have pointed out.

        Anyway I’m waiting for the AD. Specifically, there may be failure propagation thereby jeopardising the fail-safe load paths in the wing/fuselage join.

    • There’s nothing in this article that proves that the wing root joint is failsafe. Just because it hasn’t crashed doesn’t mean it can take limit load. If it can’t take limit load it is not failsafe.

      • I have been told I trust Boeing engineers too much. I just don’t believe they can be that wrong.

      • I should have given a better answer. Regulations don’t allow a single point of failure in an aerostructure. Regulations make clear aerostructures must be fail-safe.

        The Comet disaster provides the history.

      • In most normal limit load tests, the failure is not where a casual look would lead eg the lower wing panel due to too mutch strtetch- as most metals are stronger in compression than in tension.

        But of course the casual look analysis is wrong.

        Both the 747 and the 777 wing tests failed in upper panel compression.

        The most spectacular video is

        While not obvious – I have it on credible source that the failure was near where predicted and on the top wing panel

        Ther 767 did not fail the wing area, but due to a missing frame near the tail near a hold down, the aft section twisted- stopping the test- but it was close enough- since destruction is NOT a requirement.

        What descriptions are available of the pickle fork cracks ***seem*** to infer an area near the body section join to the fork.

  22. Of intersting is a quote from Muilenburg on the 737MAX

    “Muilenburg said he doesn’t expect the structural shift to substantially change the day-to-day work of those carrying out design and testing. But connecting the technical experts to other engineers across the company will create a new, informal network they can consult when problems crop up, he said.”

    I think a formal link is the right way to go, informal means its optional and we should not leave this to possible falling through the cracks.

    In other words the network should have a chat room that discusses all the day in day out issues so that someone can trigger on an issue they are seeing and comment or respond

  23. Good backgrounder from 2012 on 737 design issues, which up to that time were fuselage related rapid decompression.
    There was a Taiwan airliner that was lost in flight from that cause, but that was seen as caused by the higher corrosion from the cargo. The other decompressions ( 1 Aloha , 2x Southwest) , the planes survived.

    It also addresses the fuselage testing done for the NG
    “Boeing took a standard 737NG fuselage off the 737 assembly line at Wichita, Kans., and tested it to the breaking point. The airframe was pushed through the equivalent of 225,000 cycles (three times the assumed safe life of 75,000 cycles for the NG series) on short duration flights—exactly the way Southwest, for example, uses the 737.”

    But there is a proviso:
    ‘, was that the test fuselage did not represent the realities of everyday flight. It lacked a wing box, the core load-bearing part of the wings where they meet the fuselage, and also the landing gear, which transmits particularly forceful stresses to the fuselage on every landing. ‘

    • @Dukeofurl: “The test fuselage did not represent the realities of everyday flight.”

      What was likely missing is the winglets which were obviously added after those tests were carried out. They do add some weight to the wing assembly that would be transferred to the fuselage with lots of torque that would accelerate the pickle forks fatigue. For I don’t think the fuselage was designed with the winglets in mind.

      • It says a fuselage test only without even the centre wing box.
        The pickle forks join the wing box to the fuselage frames.

        Just goes to show how old the design is . Modern planes have the centre fuselage section and the wing box are built- up as an integral unit .
        We dont have the line numbers for these planes, so dont even know if they had winglets originally or even if they were added as ‘after market’

  24. I see where it has been noted that these cracks were not found on D-Checks. When they inspect for these pickle fork cracks, will they be able to do it quickly or will airplanes be out of service for a few days? If they can rotate their fleets, then it should be manageable; if not, SW, Ryan Air, lots of others are going to be looking for MD-80s and Classics in the desert. I’ve already read that some B737-200s have moved from charter service to regular service.

    • Not 200s with the JT8 engines , but still quite of few of the late build (
      up to 1999) 300/400/500’s still around

  25. Apparently its been pointed out the ‘pickle fork problem’ was quite extensive of the 767 series too, although here they are referred to by their technical name–300–300f-and–400er-series-airplanes
    Someone provided a photo in 2013 on facebook of the 767 picklefork

    • Fabrication and assembly issues for these and other critical parts was reported before on the NG. Honestly I am not surprised that this is popping up now, as the whistleblowers said it would. I know it sounds like a conspiracy theory but given Boeing’s behavior lately it really reinforces the thought that they would shut up on issues that would cause them more losses. The fact that it was a customer and not Boeing the one that found the issue should be alarming to the industry.

    • Good picture of the part in question. Actually, great picture of the B-17 in this sequence. Since there is only a handful of B-17s today, one has to wonder if that may be the one that crashed on the East Coast earlier today.

  26. I read elsewhere that the pickle forks were changed from milled out of a solid block originally, to forged in a rough approximation so final milling did not waste as much time and material.

    If true, was the new production part subjected to full lifespan stress testing (3X stress cycles or similar)? And all other validation processes to confirm the part performs as designed? And at a sampling rate large enough to statistically represent the full range of manufacturing variation? Plus everything else I’m missing?

    Not a super simple process. Wonder how far it went.

    And by the way, I’m also curious about the same for the new pickle forks and entire wing load transfer structure in the MAX, given the extreme time pressure for the MAX development, with the appearance of leaning heavily on grandfathering.

    • Very good questions !

      We can only hope that the various regulatory bodies are asking the same questions, and hopefully being satisfied by the answers.

      Does anyone here know how the wings of the 777 are attached to the fuselage, and for that matter, the A320, A330 etc ?

      Sadly I don’t think we’ll see BA reverting to an engineering driven company, I suspect that sales/marketing, and the stock price are firmly in charge.

      • ” Does anyone here know how the wings of the 777 are attached to the fuselage”
        YEP -generally the same methods and design used on previous 767 etc, BUT with more precise tooling and fitting due to extensive use of CAD. As to how sell that worked, see the famous wing test

        And the post debrief quick look data given within about an hour after the test established that the failure in compression ( top surface ) was within a few inches of predicted location and a few percent of predicted load and essentially on both sides at the same time. I was there and at the debrief

          • Good pics.

            Notice the circular fuselage frame continues down to bottom of wing box and isnt a separate section- connecting frame and box. Doesnt seem like the frame is replaceable like a ‘pickle fork’ would be ?
            From pictures of the 737 fuselage it seems that the structural wing box is made as part of the wing and they are joined on the centre line of fusellage as two spans. ?

          • re Dukeofurl
            October 3, 2019 and wingbox. The box is a continuous box extending from left side body to right side body ( approx) where it is then attachd to the wings along the top and bottom ( waterline ) edges generally along what is known as a plus chord . Thus, except for access plates, the box is a complete integrated structure and by sealing can be used as a fuel cell. Thus the wings are not joined ‘along the centerline ‘..
            I’m sure someone can find a suitable illustration- but pics of the train deailment duming a few green coated bodies in the river show a yellowish ‘ panel ‘ below the two ( white covered) cutouts for doors. Those ‘ panels’ are the outer side of the wing box and essentially symmetrical right side to left side.


      • Well its not a matter of hope its a matter of fact

        And no, a produi9on process change is not required for a full up test, the engineering process and procedures and material are reviewed, subject to localize test or data verification its good and put into service.

        Sometimes its a shift from hand built to industrialization (GE and the rather spectacular engine shaft shed GENx on the 787)

        I do think those critical changes need more in depth as its far from the first time this has occurred on engines though its much less common on a fuselage or wing. P&W has had a number of those and RR with the Trent is still behind the 8 ball. Those are all engines in a highly stressed environment.

        What the non tech type don’t get it that all aircraft undergo a constant process of changes. Some are efficiency improvements, some are as a result of tests and found to need changed, some reduce weight and some are aerodynamic improvements.

        If you tested each aircraft for each change, you would have a billion dollar aircraft.

        So there is a balance and its not Boeing or Airbus, its all aircraft mfgs.

        Delta reports NO issues have been observed in the area indicated on its aircraft.

        What that says is its inspected area. Likely most operators are the same.

        So we have two basic questions .

        1. Why were these aircraft not inspected correctly (or were they and it showed up between correct inspections) ?

        2. What flaw led to the crack

  27. Could this result from the airline regularly overloading the aircraft? If so, perhaps it is not an engineering or production issue.

    • Right now anything is possible.

      This is either only occurring way down the road and missed or manifested between inspection or a varying set of possibly these were not inspected correctly or they were abused (though they should take abuse)

      The intent of the way past any possible aircraft lifetime is to reveal stuff like that.

      Its one that needs more data and with more data the issue will clarify.

    • No . The design will be tested to far exceed any loads expected during certification.
      Even when building each plane on the final assembly line they undergo a pressure test , which is equivalent to the fuselage flying to 90,000ft.

  28. Anyone know the AD number – I can’t see it on the FAA or EASA websites.

  29. I am glad the issue is being dealt with FAA must always ensure aircraft manufacturers do the right thing safety first.

  30. There it is, posted twice in the last few comments. 1911 B737 NGs need to have their pickle forks inspected. And 165 of these planes have to be checked right away. I would think this would mean flights will be cancelled. Also, mentioned along the way, would this be as big of story if it hadn’t been for the MAX problem.

  31. And now we also hear the “bear straps” require inspection on most 737s…
    If I understand correctly these reinforce the fuselage at door openings and would be subject to cracks.
    Is there no end to this drip-drip-drip of technical issues?

    • There are a plethora of ADs and required inspections on ALL aircraft all the time. 100s.

      As soon as one problem is noted (and this is clearly serious) then the press jumps in and replacing a bulb in the Lav becomes a crisis.

      As you get real data then you can make some judgements as to a problem (or not in most cases)

  32. Yeah, dosnt seem so bad now,if these 165 aircraft fail at the same rate as the freighter conversion it would be about 30.Southwest 700s might be less susceptible, being lighter. 1000 cycles for the rest is more than 6 months.Its interesting that only 20 737 NGs are not required to have the checks,I wonder why?

  33. Other posters on here have said there is more to come, it seems they are correct:

    It’s worth reading through.

    “They show the federal regulator struck out four separate clauses that would be requirements for any new jet being produced today.”

    “In addition, those fixes Boeing developed after the three crashes are not necessarily installed on all the older 737s now in service globally.”

    Grandfathering really does need to change. We’ve learnt safety lessons over the years, quite often at the cost of people’s lives, and yet we still make exceptions due to perceived cost.

    Perhaps BA should put Curtis Ewbank in charge of aircraft safety at Boeing, reporting directly to the CEO.

    • Somr great articles coming from the Seattle Times as well as other news agencies in America. This one is up with the others.

      So we now know that Boeing and the FAA knew about the bedlam on the flight deck caused by a FCC meltdown whenever a sensor went wrong. It was my first comment after the Lion Air crash after reading the FAA’s emergency AD. Specifically, the FCC went into meltdown.

      The excuse for not fixing the FCC meltdown isn’t valid. As one American new agency said, the airplane hijacked itself. Great analogy. But, specifically, the FCC hijacked the airplane because the alpha vane failed.

      The same happened with the Turkish Airlines crash in the Netherlands. The FCC hijacked the auto-throttle because the radio altimeter failed. The airplane was a 737 NG.

      The current proposal of Boeing is not to allow the pilots to turn off MCAS. So MCAS can still hijack the airplane. But then we come to the auto-pilot. Pilots can turn it off, but sometimes we are told it doesn’t actually happen. The auto-pilot remains engaged.

      The article says Boeing put a price tag of $10billion on re-engineering the flight deck and therefore the FCC. That’s why the FAA gave them an exemption. As they were making $4-5billion a year out of the 737, that’s not a good enough excuse. Anyway, $10billion is far too high.

      I sure there is still more to come.

      • Like you, I’m fairly certain that there is a blockbuster of a revelation still to come.
        To be fair to Boeing, the MAX actually seemed to be genius in the circumstances they found themselves in at the time. But at some point they must have realised that they had walked into a trap,certainly after the Ethiopian crash.

        • There seems to be a narrative that Boeing had no choice but to do the Max because existing large single fleet customers would not buy a new aircraft.

          That’s not the case at all. Rather if Boing had build a NSA the likes of Southwest would have evaluated it against the A32x and the A220. So yes Boing would have lost some of those sales. But they would also have won a fair number.

          Boeing had a choice, they just chose to chase the short term revenue with one more derivative rather than build a new aircraft that could be developed for the next 30 years or more.

      • Don’t forget the WSJ article which reported that a synthetic airspeed system was left again for cost and commonality reasons.

        And if the Max seems old now, consider that based on current plans it will still be flying passengers in 30-40 years.

        • As you say, it could still be flying 30 to 40 years from now, so why didn’t they fit EICAS ?

          “Installing EICAS on the 737 would be challenging,” apparently, but perhaps not that challenging as EICAS is fitted to the P-8A, and that’s based on a 737.

          Then again MCAS on the KC-767 seems to have been done better than MCAS on the MAX, why the disconnect ?

        • FAA Federal Aviation Authority
          FCC Flight Control Computer

          Can’t see what’s wrong

          • Only that your negative diatribes add to the confusion of what the FAA CONTROLS versus what then FCC CONTROLS.

            And Whom ( organization versus computer ) is responsible for what

          • @Bubba

            Did you see any error regarding FAA, FCC or any in that what @Philip wrote? I don’t see.

          • groooan … Pablo
            October 6, 2019

            Did you see any error regarding FAA, FCC or any in that what @Philip wrote? I don’t see.


            AFIK the AOA sensor did NOT rely on anything but maybe timing from the FCC. IF flaps were up and autopilot off, it worked. Therefore just turning autopilot off ** apparently ** would NOT have stopped MCAS. The ONLY way to stop MCAS was to turn off all power to ALL eletrical.

            But for more detail start here


          • @Bubba

            By your answer with technicalities with this or that I deduct that @Philip didn’t confused nor what FCC (as whole computerised directed automation) in general do, or FAA in general do.

    • Thanks for the link. Excellent article but no good news for the 737.
      Instead of getting closer getting back into service, this offers more fuel to the theory that Boeing has indeed sacrificed safety for profit.
      There are probably mostly great people working for Boeing, but the company culture is rotten. I’m not sure that can be fixed without going belly up first.

  34. Like you, I’m fairly certain that there is a blockbuster of a revelation still to come.
    To be fair to Boeing, the MAX actually seemed to be genius in the circumstances they found themselves in at the time. But at some point they must have realised that they had walked into a trap,certainly after the Ethiopian crash.

  35. As far as I can see all affected aircraft are from the one airline. These won’t be the only f conversions, nor the oldest. I can’t see maintenance being an issue, is there any on this part? Can this be related to usage? Nasty airports, short runways or something like that?

  36. New engines,enhance the cockpit and controls of the safest airliner ever to fly.Whats not to like without the benefit of hindsight?

  37. I may have overseen this:
    Which commerical entity did begin the conversions and in that process discovered the cracks?

    I’ve found two offers for conversion of NG frames: IAE and Boeing ( 737-800BCF )

  38. The size, location and function of the “pickle fork” might make a repair rather difficult and expensive. To replace them (all 4?) you would have to rip open pretty much all of the center fuselage…

    As this is not a “usual” repair, I assume that there is no repair method tested and approved. There are probably no replacement parts available either. Jigs and all sorts of tooling would have to be designed and built. This could take months and would involve massive costs.

    So the question is: Would it be feasible or would those planes be scrapped? This whole affair might put airlines that rely heavily on the NG (and MAX) put in a really difficult position.

    Who will have to bear those cost / losses? The airlines or Boeing?

      • That’s what we were after. Not being an expert, I am a bit confused how one hole can have an enormous crack and the other 3 are OK.It appears that the attachment hasn’t actually failed and its quite comforting to see how it hasn’t come unzipped with the extra load on the remaining pins.

        • Still, if found on an NG( or an other) specimen that will turn it unflyable. Instantly. .-)

      • Great find Bubba.

        The rate of spread of the cracks will really determine what happens to these aircraft. It doesn’t look like it’s very easy to access this area of the aircraft to make repairs.

        Figure 1 Sheet 1. on page 9 is very interesting, note the ‘No damage is allowed at these locations’ legend, the cracks appear to be right on the boundary.

        It’s also interesting that only STA 663.75 is to be inspected, i.e. just the aft pickle forks.

        Does this mean that STA 540 on their subject aircraft has been inspected, and found to be crack free, or can BA predict from their load modelling that the aft pickle forks would always crack before the forward pickle forks, and thus would not need to be inspected ?

      • Good pics. Looks to me quite bad. Not 1000 cycles for airplanes above 30,000 cycles. It’s 7 days.

        The 30,000 cycles may reduce if there are lots at 30,000 cycles.

        The AD says each examination must be reported and immediate repairs are necessary.

        Not the end of Boeing.

    • Gundolf,

      It will cost and perhaps with older airplanes the decision may be made to part out the airplane. But it can be done. How much, I don’t know.

      Secondhand values will be reduced becuase of the problem. So the secondhand market will be affected.


      The language in the AD gives the insight. The language is always controlled. But the lanaguge is severe.

      (e) Unsafe Condition.
      “This condition could adversely affect the structural integrity of the airplane and result in the loss of control of the airplane.”

      Those words are rarely used, they are severe, and not far off requiring an immediate grounding of 737 NGs with 30,000 cycles or more.

      It should have been caught much earlier! But at least it’s been caught.

      • @Philip: “Those words are rarely used, they are severe.”

        They are indeed severe but are frequently used on an AD (Airworthiness Directive). That is what actually sets apart an AD from an SB (Service Bulletin); the latter being usually less restrictive while the former normally requires immediate attention.

        The difference is in the name: a “directive” requiring immediate response because it’s a major safety issue versus a mere “bulletin” telling you what you need to do to improve your airplane and possibly make it a little safer while not being particularly urgent.

        • From my reading of ADs, in order of severity

          ‘Reduced control’

          ‘Loss of control’

          ‘Loss of airplane’

          I’ve read a lot of ADs. Overwhelmingly the language is the first. I’ve only seen the last once, when the GENX shafts disintegrated. In between, for me it’s rare.

          But then a could be reading the wrong ADs

  39. Does anyone know what a brand new Pickle Fork might cost if purchased from Boeing?

    • Guess about $3 with boeing logo $10 if silver plate 5 cents if plastic with boeing logo and 2 cents if no logo

  40. Boeing is in trouble so now they want to kill of its only competitor to offset the damage.

  41. Interesting that the lower fasteners where the cracks are look to be some kind of swagged nut or even a fancy rivet.

  42. One factor is whether or not the end rib of the center fuel tank is at the fuselage join point. IIRC one airliner model achieved a span increase by moving it outboard, IOW new center wing.

    Finding cracks that early is of substantial concern. Hopefully inspection schedules and areas are complete enough to catch such surprises.

    • On the A350 the center tank reaches into the wings.
      but that is fluids path not force.

  43. I didn’t think much of this at first .. but, from the aviation herald comment section..
    First 10 acft checked 5 cracked
    By Jetson on Saturday, Oct 5th 2019 19:41Z

    My company started the AD checks two days ago.
    By the A.D. Aircrafts with 25.000 or more cycles have 7 days to have the pickles checked otherwise will be grounded.

    Aircrafts with less than 25.000 cycles have longer schedules.

    In 5 of 10 first aircrafts checked (all with more than 25.000 cycles) the cracks were found and are grounded. They don’t have any idea for how long since there is no definitive bulletin to repair it (the A.D. only states which aircrafts will be checked first and to have all aircrafts with cracks grounded until further instruction)

    They are guessing they will be grounded for more than 40 days.
    If they have to ground large numbers of 737-NG’s .. with no current repair procedure .. Boeing is really in a pickle, along with airlines that have them grounded.

    • The trigger was set by finding cracks on 3 airframes during conversion preparations. Number frames you start conversion on
      is as far as I can guess “limited”. i.e. it is 3 from a set of 4..6 ?
      thus it is not unsurprising that the hit rate is high even for
      less used frames.
      This will have unpleasant and far reaching fall out.

        • Wasn’t it 3 which were under conversion to freighters , while all the rest checked were just in the shop for various reasons.

        • someone ( avherald comment section ) posted 50% hit rate for samples upward of 22k cycles and for his airline.
          3 out of 15 for older frames doesn’t quite jibe?

          • We’ll know within a week, they won’t be able to keep it quiet if significant numbers are involved.

  44. 50% fail rate on inspection is a strong sign there is a design issue rather than materials/manufacturing.
    B needs to develop a repair SOP in a rearing hurry, or SW, Ryanair Etal will have to switch to buses (and I don’t mean Airbus!).

  45. If there is an issue with this many hits (still early) it is not a design issue (directly )

    The design has passed a far more strenuous test

    It points to an industrialization issue where the design stayed the same but the method to mfg the i9tem was changed.

    Still too early and second hand (no name or company and for who) is not the same as reported input into the system.

  46. @Bubba: “hey norm hamel ?”

    You are addressing the wrong individual. That comment was made by @A Jones, not me. I was only referencing the video related to this topic.

  47. I saw this entry in but I’m sure isn’t a bogus – a poster was referring to 25.000 cycles but AD states about 30.000 cycle for express inspection.

    It could be a design problem if encountered in so many aircrafts or manufacture one, or any. Someone tried to find out if they in Boeing tested also pickle forks with a fuselage or did it right, but without a final success.

    We need to wait for a raw data. I hope it will come publicly in a next week.

  48. I wonder how many times the airplanes may have been flown seriously over max gross???

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